The short answer would be, “you can’t.” No, Christian Bale growling at the camera doesn’t count. The problem is Batman is a very formalistic character, so pushing to the other end of the spectrum is going to require something gets tossed.
Or you can give your not-batman superpowers. It may sound slightly insane, but a character like The Shadow is, in many ways, more realistic than Batman, even though he’s literally a pulp era superhero. (And, incidentally one of the major inspirations for Batman.) The Authority’s Midnighter is another potential example of a “realistic” Batman, though, again, because superpowers make it possible.
The problem with a realistic Batman starts in the day. Specifically there aren’t enough hours in it to allow someone to do what he does. Maintaining a cover identity is a time consuming process. While billionaire layabout might seem like a pretty good gig, it actually involves a lot of micromanagement. He needs to show up to enough social events and make a mess of himself enough of the time to both convince people that he really is Bruce Wayne, and there’s no way this derp could possibly secretly be Batman.
I won’t pretend to be original in pointing out that this also means Bruce Wayne will habitually seduce a woman, leave a party, get a couple blocks from the event, feign a headache, drop her on the sidewalk, and then scamper back to the batcave, get changed, and then go hunting criminals for the rest of the night, before tormenting and then pounding on them. And he does all of this without developing any kind of a reputation for habitually dumping high society socialites on Gotham’s crime ridden streets.
Also, he does this without tipping off the paparazzi that something is seriously strange with this guy. I get that this wasn’t an element of the world when the character was originally created, but it also puts a time limit on how frequently he can duck into the batcave and drive off into the night before some freelance photographer realizes that Wayne Manor has a secret back exit for superheroes.
While we’re on the subject, I should probably also link the Cracked article about bodies being buried under the Batcave.
After that, he spends what’s left of the night hunting petty criminals and super villains indiscriminately. Returning to the batcave randomly, in an attempt to sort out whatever bits of evidence he’s found and anticipate some criminal conspiracy or deranged supervillain’s master plan. (We’ll be coming back to this in a minute.)
Depending on who’s writing the comics, Gotham’s nights have a nasty habit of being infinitely long. Thematically it’s fine, but since we’re talking about realism, it’s worth pointing out.
He sleeps for a few hours, and then staggers out in the afternoon pretending to have a hangover. And then goes and does it all again.
Until the sleep deprivation starts affecting his ability to fight, which would take roughly a day.
The second problem is, ironically, sidestepped by DC. Note: I didn’t say “intentionally.” We’ve gone over how much wear and tear fighting causes in the past. When you’re talking about a character who’s going out and roughing up street level hoodlums on a regular basis, he’s not going to be able to keep that up for any serious length of time before his body simply starts falling apart. This means he has, at best, a few years of crime fighting in him before his body is completely wrecked. But, ignoring things like Batman Beyond or Dark Knight Returns, Batman hasn’t aged since 1939. Of course, that also implies he’s going through Robins like popcorn.
This is assuming he doesn’t make mistakes and get injured, which will happen because he’s sleep deprived. When he does get injured, that will force him out of the action for weeks to months, depending on how serious it is. Except, we frequently see Batman going right back in immediately, on force of will alone.
For those of you who’ve never experienced a serious injury first hand… I envy you. Also, serious injuries basically come in two flavors. Ones where you literally can’t use the injured body parts, and ones that are waiting for you to ignore them so they can turn into the former. Trying to force your way through the pain is also a fantastic way of turning a minor injury into a permanent one.
Even if he somehow manages to avoid direct injury, the wear and tear of constant combat will still destroy him. Stress fractures are the first thing that occur to me, though that’s not the only possibility. Batman’s particularly vulnerable to nearly every common sports injury associated with leaping off of buildings at random. Which includes the day when his leg shatters in a spiral fracture.
And we’re not done. Batman’s “The World’s Greatest Detective,” because, honestly, when you’re a superhero, why not go all out for your business card? But, the problem is, he really can’t investigate a crime without muddling the whole thing up. In theory, that’s what he’s doing, investigating crime… and making sure The Joker goes free?
There’s three problems. First, Batman can’t testify, (unless he’s Adam West). Second, Batman’s “interviewing” technique is going to be inadmissible. Third, any evidence Batman touches can’t be used to prosecute the supervillains and petty criminals that ravage Gotham.
By nature, you can’t really put Batman on the stand. Who do you call? Where do you send the summons? Does Batman make himself available for depositions during business hours? How do you know the Batman who roughed up the defendant and left him hanging twelve stories above the city is the same Batman you managed to put on the stand? How did the police even know what the defendant even did? And, finally, “who is that masked man?”
Are the police supposed to simply incarcerate someone on this random animal themed lunatic’s say so in a city overrun with similarly themed lunatics running amok?
To prosecute a criminal (it doesn’t matter if they’re The Joker or a failed wannabe mugger from Burnley) you need to establish what they did. That involves opening up a large chunk of the police investigation and picking it over. If 90% of your work was done by Batman… that doesn’t really work anymore. It doesn’t matter if The Joker tried to nerve gas half of Gotham, if you can’t prove it court, you can’t lock him up.
Even if he confessed to Batman, that’s going to be inadmissible. It turns out dangling people by their toenails from the 23rd floor isn’t an accepted form of police interrogation. Suspects have civil rights. For example, the right to not be tortured by a deranged psychopath, and mock executions, like pretending to drop someone from a high rise, are torture. And, once one of the people investigating the case has started torturing the suspect, it taints the entire case.
Incidentally, that also leaves Batman open to a civil suit. Yes, that’s the plot from The Incredibles. Yes, it’s funny. Yes, it’s a real potential, and not even just in spurious litigation. Batman beats the living tar out of people on a night to night basis. If the prosecutor fails to get a conviction because Batman tortured the suspect, that’s a very plausible tort.
While we’re on the subject of technical legalities, Batman needs a warrant. This might sound insane, but if he’s investigating crime for the police, even if he’s pretending that he’s doing this on his own, he’s acting on their behalf. (I think the technical term is, “acting as an agent of the state,” but it’s been a long time since my pre-law classes.) Which means he needs to follow procedure, more or less. So, he can’t just break into a warehouse and rifle through the place looking for contraband. Or, at least, if he does, he can’t use anything he finds to further his investigation.
As a quick aside, when Batman was originally written, this wouldn’t have been an issue. Used to be, police could accept evidence from an anonymous informant, and the chain of evidence would start when the officer took possession of it. This led to “black bag jobs” where law enforcement officers (the early FBI was particularly fond of this trick) would burgle a suspect, make off with evidence, and then pass it to themselves, start the chain of evidence, and never request a warrant. So, today (well, any time after 1972), Batman needs a warrant.
That’s the other side of what happens when 90% of your work was done by Batman, then he’s been handling your evidence, turning it over, poking it with a stick, and trying to make sense of it. Sometimes this means he’s just staring at it, but a lot of the time it involves taking the evidence with him and conducting his own forensic tests on it. This is a huge problem.
In a criminal prosecution, maintaining the chain of evidence is vital. It’s not enough to know that “this bullet matches the suspect’s weapon.” You need to also be able to prove that the bullet is the same one the police dug out of the wall originally. This starts the chain of evidence, and everyone who handles it has to sign for it. The original evidence will be marked in a way to make it easily identifiable by whoever collected it originally. If it needs to undergo forensics testing, that is done under scrutiny. Wherever it goes, there is a trail of signatures for everyone that touched it. Again, this is absolutely vital, because the prosecutor needs to prove that this bullet matches the defendant’s weapon and that it is the same bullet a member of the GCPD pulled out of a wall in the Diamond District.
If Batman took the bullet off to the Batcave and came back with one that matches this gun he found… how do you know it’s the same bullet the GCPD surrendered to his custody in the first place? Any half-sentient defense attorney could tear apart any case Batman so much as touched. Which might explain why, half the time, Harvey Dent wants him dead so badly.
How do you make a realistic Batman? You don’t. Realism sabotages the fantasy that sustains superheros. Most of the time, when someone says they’re going to do a “realistic superhero,” they end up with either a retread of Watchmen or a bucket filled with gore and leftover ‘90s edgy.