The reason Stallone did a decent job in the original Rocky is that he was the character. In other words, he was playing himself. I'm not suggesting he was ever a collector in real life but the whole persona wasn't too different from how Stallone was before hitting it big. You know how certain actors essentially play themselves in every role they are in, like Clint Eastwood or Henry Fonda or Jimmy Stewart? Well, that is Stallone. He can't play another role. He is Rocky. That's how he was able to write the script in 3 days. He's not really an actor. He just acts in a movie, being who he is, on or off the screen.
The first movie won the BEST PICTURE Oscar at the Academy Awards and started a BILLION-dollar franchise. So I think the casting was just fine. You can't do better than winning Best Picture and creating a Billion-dollar franchise. By the way, the movie was filmed in 1976. The other OPTIONS for the lead when the movie studio agreed to buy the script were Robert Redford, Ryan O'Neil, Burt Reynolds and James Caan. How many Rocky movies do you think they would've gotten out having Reynolds or Caan as the lead? Which one beats Creed, Clubber Lang, Drago or Antonio Tarver? https://tse3.mm.bing.net/th?id=OIP.2pT8rG8CBMaKKHv4fP4tgwHaJM&pid=Api http://movie.info/actor/James_Caan_1.jpg It's a B-movie with anyone else.
People forget all the advances the first film made. Stallone fired the guy the studio brought in to choreograph the fight scenes. He was an old school studio guy who would choreograph barroom brawls for westerns. Then Stallone and Carl Weathers studied the Ali-Frazier films and choreographed their own fight scenes. It was also the first film that utilized portable cameras strapped to camermen on harnesses that didn't bounce when the camermen moved. It enabled them to film the iconic running sequences and allowed them to move freely around the ring during filming of the fight scenes.
This is a great site that discusses a bunch of scenes cut from the first Rocky movie. This is one of my favorites. Dipper (the guy who takes Rocky's locker) actually had a much larger role but was edited out of the final cut. He actually wanted a shot at Creed but Rocky got it, so Dipper is angry. In one scene, Creed comes to Rocky's gym with a camera crew to drum up publicity and Dipper challenges Rocky to a fight. The scene also shows how the black fighters in the gym resented Rocky for getting a shot, since he was a bum (but white). The studio cut this because they didn't want the movie to get racial and they wanted to portray Rocky as someone everyone rallied behind. The script for the scene and photos are included here. http://theeditroomfloor.blogspot.com/2012/04/lost-scenes-of-rocky-part1.html?q=rocky
Rocky 4 is the most fun. 3 is probably the best sequel overall in terms of a movie. 2 was too predictable. Ebert recalls watching the movie with Ali in his mansion. Ali pretty much guesses what happens. It's a good read for those who haven't seen it yet. https://www.rogerebert.com/interviews/watching-rocky-ii-with-muhammad-ali