Hollywood's actors and writers will join forces on the picket line from Friday after studios failed to reach a deal this week with the Screen Actors Guild (SAG-AFTRA).

It is the first time that the two unions have been on strike simultaneously since 1960 when actor (and future US president) Ronald Reagan led the protests.

What does this "double strike" mean for Tinseltown?

- Will the big stars strike? -

Among SAG-AFTRA's 160,000-strong ranks are many of the world's biggest stars.

Hollywood's A-list, from Tom Cruise to Angelina Jolie to Johnny Depp, are card-carrying union members.

Celebrities including Meryl Streep, Ben Stiller and Colin Farrell have come out publicly in favor of a strike.

But will we see them on the picket lines?

"There will be visibility from the big stars," said entertainment industry lawyer Jonathan Handel.

"But this strike is not about bringing more money to people who already have millions."

Top stars do not stand to gain financially from the strike, because their agents negotiate individual contracts with studios that far exceed the union minimums being fought over.

Still, their presence can "shine a light more on the studios, to come to the negotiating table with a fair deal," said actor Dominic Burgess.

- How will it impact movies and shows? -

Hollywood productions have already slowed down significantly since the writers' strike began in early May.

Shows with finished scripts, such as "The Lord of the Rings: The Rings of Power," were able to continue filming this summer, though without any writers on set.

But without actors, the only US-based productions that can continue are a handful of soap operas -- which have a different contract -- and reality and game shows.

For this reason, Fox this week unveiled a fall television schedule full of unscripted series such as "Kitchen Nightmares" and "Lego Masters."

Movie releases are less immediately affected, because of the long lag between the end of filming and the start of screening in theaters.

But the longer the strike goes on, the greater the impact on movie releases.

Major Hollywood studios have already reshuffled their release calendars. For instance, Disney recently pushed back several Marvel superhero films, spreading them out across a longer time period.

SAG-AFTRA has suggested it could offer waivers to exempt smaller, truly independent films.

- Will overseas productions be affected? -

SAG-AFTRA is an American labor union, headquartered in Los Angeles.

But that does not mean the impact of the strike will be confined to US borders.

"When SAG-AFTRA actors are working on the movie being shot in Europe, or Australia, or Asia, or wherever, they will have to stop work," said Handel.

The strike also prevents members from promoting TV and motion pictures, meaning that premieres and important fall film festivals such as Venice and Toronto will be affected unless the strike ends.

As things stand, "at the Venice Film Festival, if a picture was shot with SAG-AFTRA actors, the actors can't promote it," said Handel.

- How long will the strike last? -

Writers have already been manning the picket lines for 11 weeks.

But historically, Hollywood strikes have varied wildly in length -- from several months to just over three hours.

So how long will this stoppage last?

"That's up to them. We're open to talking to them tonight!" said a bullish SAG-AFTRA president Fran Drescher, at Thursday's press conference, referring to the studios.

"It's up to them if they're willing to talk in a normal way that honors what we do," she said.

Handel predicted the strikes will last at least until the fall.

"This is going to drag on, and is not easily resolved, because both sides view this as existential," he said.

"There's a lot of bitterness between the writers and the studios, and the actors and the studios."

- What is the economic impact? -

From accounting to catering to transport, countless businesses are tied to the entertainment industry.

That makes the financial impact of a Hollywood strike hard to calculate, but incontrovertibly enormous.

"Fifteen years ago, when the writers were on strike -- it was a 100-day strike -- and the estimate was a little over $2 billion. So that translates to $20 million a day," said Handel.

Adjusted for inflation, that's close to $30 million a day lost in California alone, he said.

"Believe me, our heart bleeds that we had to make this decision," said Drescher.

"But we can't not get what these members deserve, because it's only going to get worse.

"This is where we drew the line in the sand and it's a terrible thing to have to do. But we were forced into it."