Salah is in a rich vein of form, netting 15 goals in 12 appearances in all competitions.
He was instrumental in the Merseysiders' win over the Hornets back on October 16.
The Egyptian scored a brilliant solo goal in the victory, while Sadio Mane also netted and Roberto Firmino fired home a hat-trick.
And the 29-year-old forward showcased his impressive mentality after the final whistle.
He spoke to Foster on the pitch and the Watford goalkeeper was left stunned by his request.
Salah is always looking to improve his goal scoring form, and he was keen know which direction the shot-stopper would have dived if they were awarded a penalty.
"The bit I want to talk about in particular is about Mo Salah and what he said to me at full-time that made me think: 'Oh my gosh, you are so professional, so driven. You want to be the best you can possibly be'," said Foster on The Cycling GK.
"So they've just won the game 5-0. Mo Salah, I think he scored one goal in the game.
"But he came up to me at full-time and he's got his hand over his mouth a little bit because he didn't want the cameras to catch it.
"He's come up and gone: 'Ben, Ben…'
"I've gone: 'Okay, yeah? You alright?
"And he's gone: 'If I'd have got a penalty, which way would you have dived?'
"Now, as soon as he said that, I thought: 'Oh, you clever, clever boy' because we do our research. All the goalkeepers out there do their research on strikers and we'll try and find out where their last four, five or 10 penalties might have gone, for example, and Mo Salah's last five penalties he put to the goalkeeper's right.
"So I've looked at him and I've gone: 'Ah okay. I'd have gone to my right. I'd have dived to my right because you put your last five penalties to the right'.
"A big smile came up on his face, a big grin, and he went: 'Yeah, thank you. I need to know, I need to know'.
"So not content with just winning the game 5-0 and scoring an unbelievable goal and being Man of the Match, he has to know the tiny little details. In his next game, if he gets that penalty, which way he should put it.
"That's why Mo Salah, he's at the top, guys. He is the top."