Autobot Inspirations
The design for Ant-Man's helmet was based off of Iron Man's helmet and Bumblebee from the Transformers movies.

Chastain Was Out Of Hope's Reach
Hope van Dyne, the lead female role in the film, is played by Hobbit actress Evangeline Lily. Before that, Jessica Chastain (Interstellar) turned down the role because of some unfortunate conflicts with her schedule.

Boromir as Hank Pym?
Before Michael Douglas got the part as Hank Pym, Pierce Brosnan, Sean Bean, and Gary Oldman were all considered for the role.

That means that James Bond, Boromir, and Sirius Black were almost in the MCU.

In with the New
The film originally planned to focus on Hank Pym, the original Ant-Man from the comics who helped start The Avengers.

However, Pym had some questionable character flaws, such as domestic abuse, so they ended up going with Scott Lang. They still kept Hank Pym in a mentor role.

Paul Rudd Became an Ant-Enthusiast
Paul Rudd wanted to learn the way ants worked together, so he bought a large ant farm. Even after he learned what he wanted to know, he was so inspired by the ant farm that he kept it.

The Ant That Almost Was
Though Paul Rudd obviously ended up with the part, Joseph Gordon-Levitt, (The Dark Knight Rises) and Ewan McGregor (Ghost Writer) were both in the running for the part.

Adrien Brody (The Grand Budapest Hotel) also expressed interest in playing the role.

Wasp Was Almost in the Film!
At one point in the script, the superhero and Hank Pym's love-interest Wasp was supposed to be in the film.

Actresses Emma Stone and Rashida Jones were in talks to play the role.