She's at her best when 'the heat is on'. [=when she feels a lot of pressure; when she needs to finish something quickly]
She said things in 'the heat of the moment' [=when she was angry for a short period of time] that she later regretted.
The company has 'turned up the heat on' [=has put extra pressure on] its employees to finish the job quickly.
(..) But I will catch him and root him out. It won't be easy. He's a master of disguise. And undoubtedly he has gone back into hiding until the heat is off. How is he? Unlovable. Has he seen it? No. I thought you'd like to show it to him. How was Paris? (..)1975 The Return of the Pink Panther