To expand on this, what we think happens with Parkinson's disease (and parkinsonianism) is an imbalance between dopamine and acetylcholine. It makes more sense if you look at this diagram, paying particular attention to the indirect pathway. Loss of dopaminergic (DA) neurons from the substantia nigra (SNc) results in constant activation of those ACh secreting neurons, which ultimately results in inhibition of thalamus from initiating movements. Therefore, using anticholinergics help with parkinsonianism secondary to haldol.

Trihexphenidyl and Benztropine are antimuscarinics that can treat the resting tremor and rigidity of parkinsonism.

One of the tx strategies for drug induced Parkisonism or otherwise is to Curb excess cholinergic activity. Done by Benz-tropine, trihexyphenidyl (Antimuscarinic; improves tremor and rigidity but has little effect on bradykinesia in Parkinson disease). Park your Mercedes-Benz.