I don't think RDJ would have done very well with it IMO. He is seen as such a goofball at times and comes off as it. Especially imagining some of the "funnier" lines of Gravity I think may have broken the situation and fear that you felt during all those scenes. I think Clooney brought some of the charm/slight humor, but in reality had this calm stoic like presence. I think that worked better then RDJ's over whelming charisma.
I think Clooney brings a little more subtlety than what RDJ does in terms of humour. RDJ can be great but I can't help but feel he would have over did it.
 
				 
						 
 
		 
 
		 
 
		 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		