Remember the general rule that regular nouns end with a, nouns representing someone doing something end with z, adjectives end with r and verbs end with e.

Regular roots end in a consonant to which at least one vowel attaches; the 'base' vowel that attaches can vary but normally depends on which form is considered authoritative. For example, consider the regular root trim which would be described this way:

trim| /t̪rim/ (REG) Verb (~e) 1. to see. Adj. (~e) 1. visible. Noun (~ez) 1. watcher; observer. 2. (~a) an image, like a photograph or painting; by implication, lifelike, as opposed to abstract.

Note that the verb form is considered authoritative, and so the personal noun form (i.e., someone who watches) is constructed with an e, as is the adjectival form.