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John Krasinski, Emily Blunt

Keith Johnson/Bauer-Griffin/GC Images

Charlize Theron is taking it old school.

The A-lister wasn't about to let her 40th birthday just come and go, so she decided to celebrate! But instead of throwing some lavish and elaborate event at a high-end venue, Theron went back to basics and threw a casual house party at her West Hollywood residence—though it's hard to remain casual about the guest list.

One of our favorite Hollywood couples, John Krasinski and Emily Blunt (who's co-starring with the birthday girl in The Huntsman) were spotted arriving at Charlize's home on Friday night for the star-studded bash. With Blunt dressed casually in jeans and flats, the pair came with gift in hand. Other familiar faces that attended the birthday bash incluude Chelsea Handler and Johnny Knoxville

Charlize Theron

Jason Kempin/Getty Images

An insider tells E! News that guests began arriving at 7 p.m. and partied until around 11 p.m.

There wasn't loud music playing, according to the source, but the partygoers were pretty loud. "You could hear people talking, laughing," the source tells E! News. "They may have been playing games because at certain points you could hear them chanting names and cheering."

Earlier in the day, delivery trucks were seen at the residence, as well as tents being set up along with food and flowers.

While turning the 4-0 is plenty of reason to party, Charlize also recently celebrated another, even bigger life event. E! News confirmed that the actress has adopted her second child, a baby girl named August, who was born in the United States in July.

So many things to be grateful for. Congratulations, Charlize!

Emily Blunt