That is according to Chelsea club captain John Terry.
The Blues have had plenty to smile about this season, with a fifth Premier league title in sight.
While much speculation has surrounded Costa's future at Stamford Bridge this term, he has still had time to raise morale behind the scenes.
And centre-half Luiz, who returned to the club in the summer from Paris Saint-Germain, has brought back a sense of fun too.
While Diego Costa & David Luiz are said to be the Chelsea jokers, they've been pivotal on the pitch
They're the ones who are always having jokes with players
Both have been key contributors to Chelsea's success under Antonio Conte, and Terry has revealed while they enjoy themselves off the field, a serious attitude has very much been installed in the squad this term.
"Probably David Luiz and Diego Costa really [are the jokers at Chelsea]," Terry told magazine GQ.
"They're the ones who are always having jokes with players.
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Fri, February 3, 2017
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Chelsea have been training hard ahead of their clash with Arsenal
"But it’s been pretty serious this year I’ve got to say.
"Back in the day, people like Dennis Wise and ex-players that I’ve played with did stuff to people’s cars and cut people’s clothes up."
Costa and Luiz are set to start tomorrow when Chelsea take on Arsenal at home (12.30pm), where they can extend their lead on the Gunners to 12 points at the top.