As for other roles such as the Lex Luthor and Jimmy Olsen, those decisions will likely happen after decisions are made on Superman and Lois.

Source: https://deadline.com/2023/06/superman-legacy-screen-tests-clark-kent-lois-lane-1235409997/

In other words, Jimmy Olsen knew Clark Kent was Superman all along.

Source: https://screenrant.com/superman-dc-adventures-disguise-ridiculous-lois-lane-prove/

In response to a rumor that the studio had already sent out casting calls for actors in their twenties for the Clark Kent, Lois Lane, and Jimmy Olsen roles, Gunn tweeted, "Untrue.

Source: https://gamerant.com/james-gunn-superman-casting-rumor-shut-down/

James Gunn confirmed Jimmy Olsen was going to end up appearing in. Now, the filmmaker has seemingly confirmed another beloved supporting character of the Last Son of n will be appearing in the upcoming blockbuster.

Source: https://comicbook.com/movies/news/superman-legacy-krypto-superdog-dcu-live-action-debut/

The “Adventures of Superman” TV series debuted the next year, with Ms. Coates, Mr. Reeves, Jack Larson as the cub reporter Jimmy Olsen and John Hamilton as Perry White, The Daily Planet’s cantankerous top editor.

Source: https://www.nytimes.com/2023/10/15/arts/television/phyllis-coates-dead.html

They may be close allies, but Jimmy Olsen has found out that being Superman’s pal isn’t all it’s cracked up to be.

Source: https://screenrant.com/superman-10-unbelievable-moments-mongul-supergirl-jon-kent/

While Jimmy Olsen was largely absent from the DC Extended Universe aside from a single appearance in the 2016 film, Batman v Superman: Dawn of Justice, the character has typically appeared in most adaptations.

Source: https://www.cbr.com/superman-legacy-james-gunn-skyler-gisondo-jimmy-olsen-instagram/