From Rocky and Rocky II to this, his characters shifts so much.
I don't just mean how the trappings of wealth and fame might change a person, its a much bigger shift than that. During Rocky III I would think back to bits from Rocky and Rocky II and it would be like a different character entirely. Far more subdued, serious, focused and pensive in III.
Is this because of Rambo? He took himself far more seriously and its like he couldn't take a step back again?
Well, this was literally the last film Sly did before First Blood but as for the characterisation I think it's a complete coincidence that he's almost displaying the same restrained, distant persona he gave John Rambo in First Blood. Yes, you could assume this but Rocky has grown to be more educated, cultured and refined by the third movie because the money allowed him the opportunity for more 'worldly' experiences. He was very limited prior to his Championship win. I think Stallone deliberately meant for this character development to happen. Interestingly, watch Rocky V, which you might have already, and you'll see Rocky resorts back to his punch drunk personality again which I think was brought on from the damage done during the Drago fight.
I think Stallone was just trying to shift the tone of Rocky and its character away from being the gritty, inner city story and into a bigger, more universal thing. It was the 80s afterall, and in that time everything was more easy going and care free. I just assumed the character changed with the times. In the 70s Rocky was a folk hero, and in the 80s he was a comic book character.
Just rewatching the series at the moment, and yes this character shift stands out slightly bizarrely viewing them back to back. It's funny that in Rocky II they pointedly show him struggling to make a career in advertising - leading to his return to the ring - yet here we've got this suave guy doing Amex commercials!
He doesn't return to normal in Part 4. He maintains the same characteristics as in Part 3. It'd Part 5 after having sustained brain damage where we seem him revert back to his Rocky 1 and 2 personality.
I think it was just the progression of the character. Rocky was older, richer, wiser, and more mature by Part III. Adrian's character had changed quite a bit too by then.
From Rocky and Rocky II to this, his characters shifts so much
IMHO, I feel it was the natural progression of the character. Perhaps the montage sequence with Eye of the Tiger wasn't enough for you to see the proper transition, but we are seeing three years of Stallone being on top of the world and increasing his wealth, fame, popularity. And when you take into consideration what happened to him in Rocky 2 when he had just a small dose of that, it's natural for Rocky to have cleaned up his act. As Mickey puts it later,become "Civilized".
I actually watched the Rocky films this week, so they are very fresh in my mind, and since they mirror Stallone's career so well, I'm sure Sly himself was a bit "rough around the edges" during the late 70s(rocky 1/2). I feel the Rocky films are a perfect fit, even continuing in Creed. Stallone really took the time to develop and write the character of Rocky, and I respect the man greatly for that.
His regression back to his "old self" in Rocky 5 is also fitting, as he's become a bit messed up in the brain (punchy) after Drago, and returning to those same streets, same clothes, he's picking up that old mojo again.(hence why he even starts smoking again)