As Wyt mentioned in Shakespeare's proverb from the play A Midsummer Night's Dream it says :
The course of true love never did run smooth , yet to use 'smooth' as an adverb ,that is only a slang or in dialectic speech.
In TV series ,they use a lot of slang , but it is not grammatically correct and you can not use it as an adverb, while writing a comprehension or official documents etc ..
So the correct adverb is 'smoothly' ,run smoothly !
Always distinguish between the slang used on TV and correct English ! You can use the slang in your speech ,but learn the grammar rules to apply at work and in exams ( if you are taking any ) ,etc ....