What is reasonable or due diligence?

Reasonable or due diligence is " 'incapable of a mechanical definition,' but it 'connotes the persevering application [of] untiring efforts in good earnest, efforts of a substantial character.' " (Cromer, supra, 24 Cal.4th at p. 904.) Factors to be considered include the timeliness of the search, the importance of the witness's testimony, and whether leads to the witness's possible location were reasonably explored. (Ibid.; People v. Thomas (2011) 51 Cal.4th 449, 500.)