At first glance this looked liked it could be one of those lesser-spotted species, a totally athematic Cinephile puzzle.
But it turned out that there was actually a healthy dose of climate change material stashed away in several of the longer downs clues (1/20, 3,6) and their answers. Given that it’s 36C where I am today, the “big heat” theme seemed very topical.

Across
1 GEORGIAN - orgi(es) in gean. Apparently gean is the “Wild Cherry”; in the botanical (rather than orgiastic) sense.
11  CH,O,IR,GI,RL - “on both sides” equating to RL (right and left) is one of those devices that is almost inpenetrable to the newcomer. But thereafter, it pretty much unambiguous.
12 ABSENTEES - Ibsen (a for i),tees. At first I thought that “for starters” referred to the initial substitution, but I think
it’s intended as a loose definition for (golf) tees.
13 (f)ORMER
15 REF,U,GEE - I haven’t checked whether “horse”=gee is as familiar in the singular form. But I suppose it follows from the babytalk.
22 C,LANG - Confession time. The only reason I know that Cosmo Lang was an erstwhile Archbishop of Canterbury, is through playing pub quiz machines excessively in my youth.
25 KI in PAST, ANI(mation) - “fellow continental” being a blunt definition

Down
1,20 GLOBAL WARMING - A Spooonerism of a phrase meaning “birdsong at twilight”. I assume that the intended link-phrase is Gloamal Warbling, but I’m not 100% confident of a a watertight parsing on this.
3 GREEN,HOUSE,GASES - a laugh out surface reading for me. Bravo!
6 CAR,BON,FOOT,PRINT - Another effective tie-in. This might be a “semi &Lit” clue, but I’ve never really figured out exactly how those are officially classified.
7 PIU in OM
9 BOWS,ER - Slang for a petrol pump Down Under.
16 AL(l)+VANIS(h) in GM - I enjoyed “Current application” as the definition.
17 STOCKPO(r)T
21 EGG in BAR - Simple construction, but far from immediately obvious.

Apologies for a late Azed report - can’t remember my time for this, or how much I used the big red book - I re-solved from memory after prematurely chucking the original.

Fantastic puzzle from “one of us” with a very clever and very topical theme. You probably know the game where you have to change one word into another in a certain number of steps by changing one letter at a time - this puzzle re-enacts that game by showing how to transform BLAIR in BROWN in 4 easy steps. Ingenious. As if that wasn’t enough there’s also some excellent clueing and very imaginative wordplay going on, and I’m not just saying that because I know the setter! That said, there are two clues I don’t understand - 7D and 19D - and I thought 19D was a bit unfair (but see my comments on that).

Given the number of black squares in the grid and the lack of difficulty in this puzzle, I was expecting there to be a theme, but, if it’s there, I can’t find it.

This only took me about 6 minutes to solve, although I have to admit that I’m not sure about 1dn.

ACROSS

4,6 COMMON OR GARDEN

9 SITCOM - short for “situation comedy”, so “place” makes the clue a bit cryptic

10 ASPIRANT - (partisan)*

11 LEGERDEMAIN (Germa(n) leader)*

18 RIGHT-WINGER - depending on the football ground you are at, wingers may be very close to the crowd, ie supporters

23 PE(A-HE)N

25 PLAIN AND SIMPLE - not terribly cryptic
DOWN

1 HONOUR - though not sure why

2 CRYSTAL SET - (tactless)* around RY - “what links stations” - the best clue in the puzzle.  A crystal set was an early radio receiver using a crystal detector.

3 SALIENCE - LIEN in (case)*

5 M.I.T.-1-GATE - MIT being Massachussetts Institute of Technology, which I think is a University rather than a college?

12 ENG(AGE-MEN)(outfi)T

13 KINGSHIP

14  FLORENCE - where Michelangelo’s statue of David is situated

19 IRE-NIC(k)

21 F-ETA - another good clue