Does the subject of your sentence do something (She said that), or is something done to the subject (That was said by her)? The first is an active construction, the second a passive one.
The active voice is much more effective. It tends to be shorter and simpler, more natural and direct, more engaging.
Lawyers, who are often accused of being verbose and overly complicated, unnatural, indirect and anything but engaging, favour passive constructions.
Which is more forceful? I love you (active) or You are loved by me (passive)? We recommend the chocolate mousse (active) or The chocolate mousse is recommended by us (passive)? No points for getting the right answer: it’s too obvious.
With good reason, Theodore Blumberg Seven Deadly Sins of Legal Writing (2008) calls passive constructions the first deadly sin of legal writing.
There are some occasions when the passive is the better choice, but not many:
- the responsible party is irrelevant or unknown (The summons was served)
- result > responsible party (Mission accomplished)
- you want to deflect blame (Mistakes were made, as opposed to Our client made mistakes)
- for emphasis (He was shot) – although it’s easy to deaden rather than heighten effect with the passive
- to improve flow between two sentences (Bupinder is a model associate. He is consistently praised by partners.)
- to vary sentence structure? trust me (note the active voice there), the passive rarely works
- sounds better? doubtful; see examples above
- to create ambiguity and uninteresting prose? for sure
Next tip: that and which