I've heard there's the whole friendmance/rivalmance thing and I don't know if my expectations of DA2 are too high but is there a difference in scenes depending on how you interact with them (friendship/rival)? Because hot damn!
There are two ways to get this scene, and both have their good points. Without spoiling too much:
1) As everyone has said, keep flirting with him (these scenes can happen whether you are friendmancing him or rivalmancing him).
2) Be sure to do his QB (Questioning Belief) before "Bitter Pill". You don't
actually have to do it prior, but it makes for a much smoother transition, since it
needs to be done.
3) After Bitter Pill, you need to return home, and a cutscene with him will start (regardless of romance or no romance). If you take the last "heart" option, right as he is leaving, the LI scene will start.
-HOWEVER-Your other option (so long as you used the flirts in his Questioning Belief), is to end this scene with the dialogue option, "Will you follow up on the information Hadriana...." (this is not a direct quote, btw, but you should figure out easily, which one it is). You choose this option of dialogue, instead of the last "heart" option, as he is leaving. This will change how the romance scene initiates. Once he leaves, have your Hawke leave the Estate as well (I usually go sell stuff from the previous quest at this time), then return home. You will have a very nice surprise waiting for you. *nods*

Both scenes are quite similar, and they are both good. I
personally prefer the second option, for two reasons (that I will spoiler, just in case):
[spoiler]
Seems to have less weird animation alignments and clipping
For me, works better with most of my role playing. From my more serious, pragmatic types, to my goofy/shier types.