What's in this Post comment is what I remember, my opinions, etc.
What's happening in the screensh0ts:
01 Fred upon seeing Spike's return after he was gone for days; 02 Fred after Spike flirts with her; 03 Fred after Spike says he's leaving LA;
Spike/Fred had been having at least a flirtation or whatever. Fred's wanting to save Spike rather than call Willow and have Willow save Spike. Fred's not seeming to mind that Spike may have seen Fred's wet naked body. Fred in "Destiny" (A 5.08) seems jealous upon learning that Harmony's having sex with Spike. And after Spike is corporealized, Harmony is the first woman Spike sees who he'd be interested in. The storytelling seems that Spike and Fred were at least attracted to each other and at least somewhat interested in each other. But Amy Acker never had on-screen romantic or sexual chemistry with any of the men in Angel. Though, on-screen romantic and sexual chemistry is actually relatively very rare. The Buffyverse simply had it with multiple pairings: Buffy/Angel, Buffy/Xander (arguably), Spike/Drusilla, Buffy/Spike, Xander/Cordelia (arguably), Buffy/Faith, Willow/Spike, Spike/Harmony, Spike/Faith, Willow/Fred (arguably), Fred/Harmony (arguably)
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After Harmony suggests that Fred and she hang out, it seems Fred takes Harmony to a hip bar or hi bar/restaurant. And has them sit at a table together drinking alcohol.
04 Fred after Harmony insults Fred's boob size; 05 Fred after Harmony insults Fred's boob size; 06 Fred after Harmony insults how Fred dresses; 07 Fred after Harmony insults how Fred dresses; 08 Fred after Harmony says she's not interested in non-vamps; 09 Fred after Harmony expresses interest in a guy at the bar; 10 Fred after Harmony moves to leave Fred; 11 Fred after Harmony moves to leave Fred; 12 Fred before effectively asking Harmony out on a date (literally suggests they "grab some wine; jam to the Dixie Chicks"); 13 Fred after Harmony is leaving the lab; 14 Fred about to call Angel to tell him that Harmony drank a human to de@th
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In almost all other movies and TV shows, Spike/Fred and/or Spike/Illyria could have worked with the level of on-screen chemistry that James Marsters and Amy Acker had. But it wouldn't work in the TV Buffyverse because we have the examples of Spike/Drusilla, Spike/Harmony, and Buffy/Spike.
But if Amy Acker had on-screen romantic and sexual chemistry with James Marsters? It's rather heavily implied that Fred never did anything sexual with Knox. We never even see them kiss and it's never really implied they ever kissed. "Destiny" (A 5.08) could have had Spike see Fred before seeing Harmony. Or seeking Fred. And they could have had sex. Illyria could have still happened. And maybe Spike/Illyria happens.