By Army Archerd, Daily Variety Senior Columnist

HOLLYWOOD (Variety) – Ben Affleck and Matt Damon are phoning/faxing each other London-N.Y. deciding which roles each should play in their next,Castle Rock’s “Half Way House.”

It’s based on stories pals told ’em about a home for the mentally impaired. While the duo were at first going to play workers in the facility, Damon now tells Affleck he wants to play one of the retarded residents. Also set to star is Affleck’s brother Casey.

“It’s an ensemble piece,” says the Oscar-nominated (with Damon) writer, now in London for Wednesday night’s premiere of “Good Will Hunting. “I’m the only one here from the picture,” Affleck says. “Matt just wound ‘Rounders’ in N.Y.”

Affleck’s also in London acting in “Shakespeare in Love,” the Tom Stoppard screenplay based on Marc Lawrence’s original. Joseph Fiennes plays Shakespeare in the 16th century-set story also starring Geoffrey Rush and Affleck’s friend Gwyneth Paltrow. Yes it’s appropriately costumed for the period and Affleck boasts, “I have the greatest codpiece ever — it puts ‘Batman’ and ‘Robin’ to shame!”

His role: “the bombastic, raging, whoring, syphilitic actor, Ned Allyn.” Gwyneth plays Viola, who pretends to be a man (!) so she can be a member of the (all-male) acting troupe. Affleck says the period picture “mirrorspolitics of Hollywood today.”

In the next Affleck-Damon joint acting venture, “Like a Rock,” for Miramax, Affleck says, “I’m the romantic lead — and Matt is the supporting, like I was in ‘Good Will Hunting.”‘ Affleck is also set for Miramax’s “Dogma,” to shoot in Pittsburgh.

How does the young (25) Affleck judge which of the many offers to accept? “I gauge them when I read the script and say, ‘I wish I’d written that.”‘

He admits to being overwhelmed by the Oscar nomination (he’ll also present) and by the plethora of pix presented and the hefty work schedule. “But, I’ve known many a day in the (not so distant) past when I had nothing to do but go and look in the mailbox.”