Every Halloween, celebrities love to show off their elaborate costumes, often based on hit TV shows and films.

But this year, things will look a little different after actors were warned that dressing as characters from major shows and movies would break the rules of the ongoing Hollywood strike.

So outfits based on Barbie, Wednesday Addams and film superheroes are banned.

Wearing such costumes would promote content made by the studios the actors are in dispute with, their union said.

“Choose costumes inspired by generalised characters and figures (ghost, zombie, spider etc),” Sag-Aftra advised its members.

Or actors could dress as characters from content that does not fall under the strike rules, “like an animated TV show”, the union suggested.

“Let’s use our collective power to send a loud and clear message to our struck employers that we will not promote their content without a fair contract!”

Ryan Reynolds responded on X, formerly known as Twitter, by joking that he would “look forward to screaming ‘scab’ at my eight-year-old all night”.

He added: “She’s not in the union but she needs to learn.”

The Sag-Aftra advice comes as the actors’ strike, which prevents union members from doing any work for major US studios, approaches the 100-day mark.

Most production has been on hold since it began, with the two sides failing to agree a deal over demands including increased pay, a share of streaming revenues, and protection against actors’ images and voices being replicated by artificial intelligence.

The latest talks broke down last week, with studios saying the negotiations were “no longer moving us in a positive direction”.

George Clooney has led a group of A-list stars who put a proposal to the union this week in an attempt to break the deadlock.

According to the Hollywood Reporter, a Zoom call was held on Tuesday between union leaders and Clooney, Reynolds, Scarlett Johansson, Kerry Washington, Tyler Perry, Bradley Cooper, Meryl Streep, Robert De Niro, Ben Affleck, Jennifer Aniston, Reese Witherspoon, Emma Stone and Laura Dern.

Clooney and his fellow big names suggested that bigger earners should pay more in union fees – which he estimated would generate $50m (£41m) a year.

That is reportedly intended to bridge the gap between what the union is seeking and what studios are willing to offer.

A Sag-Aftra statement said they were “grateful that a few of our most successful members have engaged to offer ideas and support”.

It added: “This generous concept is worthy of consideration, but it is in no way related to and would have no bearing on this present contract or even as a subject of collective bargaining.

“It is, in fact, prohibited by Federal labour law. For example, our pension and health plans are funded exclusively from employer contributions. It also doesn’t speak to the scale of the overall package.”

The strike would go on for “as long as it takes”, they said.

“For 98 days, we’ve stood strong, united in our pursuit of justice, fairness, and the value we bring to the industry.”

A similar strike by scriptwriters was resolved last month after 148 days.