Marvel pays the rest of the Avengers cast a fraction of what they pay him. What does Robert do in response? Knowing that they are not willing to risk replacing him, he threatened not to return for any more Marvel movies unless they paid the other actors fairly. Also a Crowning Momentof Heartwarming.
Also counts as a moment for Joss Whedon, who also said that there wouldn't be an Avengers 2 if Downey couldn't be Iron Man.
Way back when, after watching The Matrix with his (producer) wife, RDJ stated, "I can do that." The rest is history.