Producers said Lennon himself would have a relatively small screen presence in Chapter 27.
Lohan's character was created to help film-makers explore the motivation behind the killing, they added.
The film is the first project from director Jarrett Schaeffer, who also wrote the script.
It is being co-produced by Toronto-based Peace Arch Entertainment and 21 Grams film-maker Robert Salerno's company Artina Films.
"It's a psychological study," said Peace Arch's John Flock.
"I wouldn't call it a sympathetic portrayal of him, but you do kind of get into Chapman's head."
The film's title is said to be a reference to the 26 chapters in the JD Salinger novel The Catcher in the Rye, which Chapman said was an inspiration for the murder.