Just seven days earlier Spain striker Costa was booed by Blues fans angry at the performances which had resulted in Jose Mourinho's sacking.
But Costa was back to his bruising, brooding best with two classy finishes capping a dynamic display which would have earned three points had Oscar not skied a late penalty.
And Hiddink, who was in the stands for last week's win over Sunderland before taking over for his second spell in charge against the Hornets, revealed the boo boys had got Costa thinking.
"His reaction was good, when he was booed he had a very good reaction as a professional," said the Dutchman.
"I think that made him think 'how should I go on?'
And he showed how he will go on today."
The only downside to Costa's performance was a fifth booking which will rule him out of Monday's trip to Manchester United.
Hiddink had told the former Atletico Madrid striker to channel his emotions in the right way if Chelsea are to salvage something from the wreckage of this season.
He responded by giving them the lead in the 32nd minute, swivelling to fire home after John Terry headed down Willian's corner, for only his second league goal since mid-October.
Watford were gifted a way back into the game before half-time when Nemanja Matic needlessly handled in the box and Troy Deeney dispatched the penalty.
Worse was to come for Chelsea after the break when in-form Hornets striker Odion Ighalo netted for the fifth successive match, his shot taking a deflection of Gary Cahill to leave Thibaut Courtois wrong-footed.
But Chelsea drew level when Costa just managed to stay onside before tucking away a superb through-ball from Willian .
"He focused himself on what he is good at and not wasting extra energy where he shouldn't waste it," added Hiddink
"He showed it with two goals.
"But he is now out with a fifth booking so we have to think how to solve that problem."
Hiddink should have been celebrating a win on his return to the Bridge after Valon Behrami recklessly upended Blues substitute Eden Hazard in the area 10 minutes from the end.
But Oscar slipped as he ran up to the ball and ballooned his penalty over the crossbar, leaving Hiddink to quip: "He must have had slicks on, when he should have had normal tyres on."
Watford arrived in west London looking for a fifth straight win and manager Quique Sanchez Flores, who played under Hiddink at Valencia, was happy to come away with a draw.
"I'm completely satisfied," he said.
"Chelsea were really motivated, very strong, very aggressive, so I am happy with the performance and really happy with the result.
"It's always difficult to play against Chelsea
They are an amazing team
And Hiddink is an amazing coach.
"It was funny being in the dug-out next to him
In the second half he told Ivanovic - who played in my position - to do something and then said 'but not like Sanchez Flores!'
Source : PA
Source: PA