Michael Keaton's casting as Batman proved controversial at the time, but his improvising of a classic line shows how right he was for the part. It may seem like a weird idea now, when Keaton stands as a defining screen Batman for many, and is now well established as a dramatic actor, but back when he was cast in Tim Burton's Batman, Keaton was more known for comedic roles. Keaton had recurred on several 1970s sitcoms, and in the 1980s, had primarily starred in films like Mr. Mom, Night Shift, and Johnny Dangerously.
Had social media been around back in the late-1980s, one wonders if Keaton's Batman casting would've sparked as large a negative response as Ben Affleck's. As with Affleck though, Keaton ended up winning over many of those same detractors with his resulting performance. Keaton was so well-received, and Burton's Batman such a massive hit at the box office, that both were happily welcomed back for 1992 sequel Batman Returns.
Tim Burton's first Batman movie is full of classic moments, but one particular line - "I'm Batman" - and Keaton's delivery of it, tends to stick out the most in the minds of fans. Fittingly, the "I'm Batman" actually wasn't scripted, and was the result of Keaton and Burton deciding to make a last minute change - and it's one that has resonated with the character ever since, even being used again in Batman Begins.
Michael Keaton gets several great lines over the course of Tim Burton's Batman, such as the wild "You wanna get nuts, let's get nuts" exchange between Bruce Wayne and Joker. However, when fans think of Keaton's Batman, it's likely one scene is first to come to mind. It sees Batman striking terror in the heart of a criminal while on a rooftop, insisting the bad guy tell all his friends about him, and answering the question "What are you?" with an immortal delivery of "I'm Batman." The line is so iconic that Keaton famously brought it back a few years ago to cap off a commencement speech he gave, to the delight of those in attendance.
When looked at clinically, it's an extremely simple, perhaps overly obvious line. When seen within its proper context in the film though, it works exceedingly well to help establish Keaton's Batman as a crime-fighter. Yet, "I'm Batman" wasn't in the script. That scene had gone through several variations on paper, including "I Am The Night" and "I Am Vengeance." Once actually filming though, neither Burton or Keaton was happy with how those scripted options sounded. So, Keaton decided to ad-lib "I'm Batman," and thankfully, both of them loved it. The rest is superhero movie history, and a crowning moment in Keaton's storied career.