An in$-$depth interview is a

interview, while a focus group is a

$($b$)$

interview.

$($a$)$ group; $($b$)$ one$-$on$-$one

$($a$)$ one$-$on$-$one; $($b$)$ group

$($a$)$ intensive; $($b$)$ brief

$($a$)$ brief; $($b$)$ intensive