In order for the black and white sections of the movie to be shot in the same quality as the rest of the film, Kodak produced a limited supply of its Double-X black and white film stock in 70mm. This film stock was chosen specifically for its heritage - it was originally sold to photographers as Super-XX during World War II and was very popular with photojournalists of the era.
Matt Damon was on a break from acting as a promise to his wife, with one condition: it would go on hold if Sir Christopher Nolan called. As luck would have it, Nolan offered Damon the role of Leslie Groves, and the break went on hold.
The Trinity detonation scene uses a combination of practical effects and digital compositing. Multiple explosions were performed practically, with a hybrid of gasoline, propane, aluminium and magnesium substances involving big miniatures. These were filmed at high speeds from multiple angles, and then layered using digital effects to create the iconic mushroom cloud.
Sir Christopher Nolan claimed that one advantage for filming an ensemble cast was that the lives and personalities of the Manhattan Project scientists are well-documented, and each cast member would arrive with encyclopedic knowledge of their character and their relationship with the team.
Albert Einstein is shown walking in the woods with Kurt Gödel. The two of them did actually take long walks together, as friends at the Institute for Advanced Studies.