Mark Ruffalo replaced Edward Norton as Bruce Banner/Hulk early on, and this fan theory finally explains why Hulk changes appearances in-universe.

A new fan theory may finally explain why Edward Norton's Hulk in The Incredible Hulk is different from Mark Ruffalo's Hulk. While MCU characters have changed actors before, the Hulk's actor change is by far the biggest example of this. While there is no official explanation, one theory (via Reddit) perfectly explains why Bruce Banner changes in the MCU after The Incredible Hulk.

Although it was intended for Edward Norton to play the Hulk and Bruce Banner in the MCU, Mark Ruffalo replaced Norton due to some behind-the-scenes conflicts. Ruffalo has portrayed the character in every other MCU appearance, becoming more associated with the character than Edward Norton ever was. The MCU has never had an in-universe explanation for this actor switch, leaving it open for speculation, but recent revelations in the MCU make some of these theories far more possible.

The MCU Phase 4 has been dabbling in the multiverse, with films like Spider-Man: No Way Home and Doctor Strange in the Multiverse of Madness bringing Marvel characters from different series into the MCU. As Loki explains, these alternate versions of characters are known as variants. Variants come from different but similar universes in which a series of events lead to massive shifts in the history of that universe. These variants do not necessarily have to be played by the same actor, with several Loki variants being portrayed by different performers. All of these recent MCU developments have led to a new theory regarding the Hulk: Edward Norton's Bruce Banner is a variant. Put simply, the events of The Incredible Hulk take place apart from the mainline MCU, in an alternate universe where the events are the same but some of the characters look different. While this theory is plausible on its own, there is actually some evidence that supports this.

The main section of the MCU that backs this theory is What If...? season 1, episode 3, "What If... the World Lost Its Mightiest Heroes?" This episode of What If...? shows the deaths of the Avengers as they are picked off one by one at the hands of an unknown assassin. Events from The Incredible Hulk are reanimated and shown in this episode, with one major twist: Bruce Banner looks like Mark Ruffalo, not Edward Norton. This lends credence to the idea that there can be universes where these events take place with Bruce looking like either actor. Another instance of appearances changing involves Abomination's different looks between The Incredible Hulk and Shang-Chi and the Legend of the Ten Rings. While Abomination looks similar in both ways, there are some changes, such as extra fins. Not every character from The Incredible Hulk looks different in the MCU, though. For example, William Hurt plays General Ross in Hulk as well as several other MCU movies. Due to how similar the story is and many of the characters are, it is entirely possible that Edward Norton's Hulk is a variant of Mark Ruffalo's.

Many timelines in the multiverse feature identical events to the mainline MCU, and the theory states that The Incredible Hulk is just another example of this. Even though it's obviously not what the creators originally had in mind, this theory would be a fun retcon to explain why Bruce Banner looks different between films. Until then, though, this is the best explanation for why Edward Norton's Hulk is so different in The Incredible Hulk.

Next: The Incredible Hulk Was Marvel's Biggest Disappointment

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