Michelle Williams in “My Week with Marilyn.”

If you’re styling yourself as an icon for Halloween, you can wear an obvious costume. For Michael Jackson, go for one sequined glove. For Elvis, don a replica of the “Aloha From Hawaii” jumpsuit. Such iconic looks work for costume parties, but they’re dangerous if you’re an actor in a much-anticipated biopic.

“My Week with Marilyn” comes out Nov. 4. The film depicts a week in 1956 when a studio assistant escorted Marilyn Monroe around Britain while she made “The Prince and the Showgirl.”

In the trailer, we see Michelle Williams donning a Vanity Fair spread worth of Monroe’s most famous outfits: a black turtleneck, a fuzzy white bathrobe, and even a sparkly gown that’s a ringer for the one Monroe wore when singing “Happy Birthday” to JFK, much, much later in 1962.

To convince us we’re getting new insight into a celeb, biopics should walk the line between authenticity and kitsch. When Williams wears exact replicas of Monroe’s clothing, she looks like a cheap impersonator, a walking postcard. I hope Williams’ wardrobe is broader than the snippet seen in the trailer. If not, she’s just another blonde in a bombshell getup.