What is the test for credibility in a murder case?

that determination depends. [Citation.] Thus, if the verdict is supported by substantial evidence, we must accord due deference to the trier of fact and not substitute our evaluation of a witness's credibility for that of the fact finder. [Citations.]" ' " (People v. Smith (2005) 37 Cal.4th 733, 738-739.)

" 'Conflicts and even testimony which is subject to justifiable suspicion do not justify the reversal of a judgment, for it is the exclusive province of the trial judge or jury to determine the credibility of a witness and the truth or falsity of the facts upon which a determination depends.' [Citation.] Unless it describes facts or events that are physically impossible or inherently improbable, the testimony of a single witness is sufficient to support a conviction. [Citation.]" (People v. Elliott (2012) 53 Cal.4th 535, 585.)

2. Analysis

"Murder is the unlawful killing of a human being . . . with malice aforethought. ( 187.) To prove one variety of first degree murderthe one that formed the prosecution's theory of the case herethe prosecution must show that the killing was willful, deliberate and premeditated. ( 189.) " 'An intentional killing is premeditated and deliberate if it occurred as the result of preexisting thought and reflection rather than unconsidered or rash impulse.' [Citation.] The reflection may be arrived at quickly; it need not span a specific or extended period of time. [Citation.]" (People v. Lopez (2018) 5 Cal.5th 339, 235.)