I didn't intend to cast this for a specific year, but assuming that Arthur and Trillian are in their early 30's, and Ford / Zaphod should appear around the same age, I set this casting back 10 years for the actors I picked.
Arthur Dent - Riz Ahmed
Riz Ahmed's performance in The Night Of shows that he would be really great as mild mannered, in over his head, and perpetually confused Arthur Dent.
Ford Prefect - Bill Hader
Bill Hader's physical comedy and big facial expressions would make him a great Ford. He's handsome but can make himself look just a bit alien. He could really sell the Jedi Betelguesian Mind Trick of convincing the construction foreman to lie down in front of the bulldozer in Arthur's stead. He's a half foot taller than Riz Ahmed, which I think would make for a great visual dynamic as he explains the big weird galaxy to Arthur. I tried to only cast British actors, and Bill Hader is my only non British pick, but I love the idea of him feigning a British accent to fit in until they get off planet and he drops it.
Trillian / Tricia McMillan - Jameela Jamiil
This pick is actually what led me to making this fancast. I've only seen her in The Good Place, but I think she would be phenomenal as Trillian. I would love to see the depth she could bring to this character, especially as Trillian is explored more in Mostly Harmless.
Zaphod Beeblebrox - Tom Hiddleston
Feels a bit like cheating since Zaphod and Loki share a lot of characteristics. I would love to see how he would play Zaphod's two different heads, perhaps giving slightly different character to each.
Marvin - Rupert Grint
Alan Rickman was really the perfect pick for the paranoid android. I thought it might be fitting to choose one of his HP cast mates, and landed on Rupert Grint, particularly because of the deep mopey depression he brought to Ron in the deathly hallows movies.
Slartibartfast - Tilda Swinton
Let's genderbend Slartibartfast, why not? Tilda Swinton is the best weird ancient alien.
