Phonetics/Phonology

What kinds of factors cause people to have a 'foreign accent' when they speak a second language? Mention at least one factor that has to do with phonetics and at least one that has to do with phonology. Give at least one specific example of a phonetic or phonological characteristic of a person's speech that makes their speech seem foreign. Do you think that teaching people phonetics and phonology as part of second language instruction might be helpful? Why (not)?