Westerhof (Westerhof v. Gee Estate, 2015 ONCA 206, confirmed the reach and extent of expert opinion from a participant expert, who is afforded an exemption from r. 53.03. That exemption is however lost to the extent the expert’s opinion is not based on the expert’s observation of or participation in the relevant events and not formed as part of the ordinary exercise of his or her skill, knowledge, training and experience while observing or participating in such events. Imeson v. Maryvale (Maryvale Adolescent and Family Services), 2018 ONCA 888 illustrates the importance of scrutinizing such evidence to ensure that the participant expert does not exceed his or her proper role. The C/A stated: “In Westerhof, this court explained the proper role of a participant witness. A who has special skill, knowledge, training, or experience; has not been engaged by or on behalf of a party to the litigation may give opinion evidence for the truth of its contents without complying with rule 53.03 where the opinion is based on the witness’s observation of or participation in the events at issue; and the witness formed the opinion to be given as part of the ordinary exercise of his or her skill, knowledge, training and experience while observing or participating in such events. If participant experts proffer opinion evidence extending beyond these limits, they must comply with r. 53.03 “with respect to the portion of their opinions extending beyond those limits”. Trial judges have the important task of ensuring that participant experts do not exceed their proper role or, if they do, that there is compliance with r. 53.03”.