crucial (esp. BrE), key, material, vital (esp. BrE)
As the last person to see her alive, he was a material witness in the case.

independent (esp. BrE)
credible, reliable
unreliable
silent (figurative)
The ancient temples bear silent witness to the passing dynasties.

appeal for (esp. BrE)
The police are appealing for witnesses.

trace
Police have so far failed to trace any witnesses to the attack.

come forward
Two witnesses came forward with evidence.

report
Witnesses reported that the suspect was a white male.

account, statement (esp. BrE)

according to witness
According to witnesses, the thief escaped through the bedroom window.

witness to
a witness to murder

chief, main, principal (esp. BrE), star (esp. AmE)
the prosecution's chief witness

potential
reluctant, unwilling
hostile
defence/defense, prosecution
federal, state, state's (all AmE)
expert
character
civilian, police (BrE)

call, subpoena, summon (esp. BrE)
The defence called their first witness.

be called as, be subpoenaed as, be summoned as (esp. BrE)
He was subpoenaed as a witness in a bankruptcy case.

appear as
She appeared as a character witness.

swear in
cross-examine, examine, interrogate, interview, question
hear
discredit
intimidate, threaten
A judicial investigation was ordered, but witnesses were threatened and none would testify.

protect

be sworn in, take the stand
The next witness took the stand.

give evidence, testify
state sth
identify sb
She was the only witness to identify Peters as the attacker.

box (BrE), stand (AmE)
summons (BrE)
testimony (esp. AmE)
intimidation (esp. BrE), tampering (esp. AmE)
The jury convicted him on two counts of witness tampering.

protection
She went into a witness protection program. (AmE)
He was placed on a witness protection scheme. (BrE)

a witness for the defence/defense, a witness for the prosecution

act as

sign

in front of witness
the marriage contract is signed in front of witnesses

witness to
Would you be willing to act as a witness to my signature when I sign my will?