I think that if Disney was confident in whatever movie it is they're making, they would have kept RDJ's involvement on the down-low, to surprise the audience. Instead, they made this big hullabaloo about RDJ coming back.

Disney famously broke the contract with Robin Williams to advertise him in Alladin. I don't think Disney would ever keep a big stars involvement 'on the down-low', regardless of how confidant they are about a movies success.