Traditionally, a Motion to Strike attacks only a portion of a Complaint, and admits all well pleaded factual allegations. It cannot be supported with evidence outside the pleadings, and if it is, courts are supposed to treat it like a summary judgment/summary adjudication motion, which almost never works because Code of Civil Procedure, § 437c's 75-day notice provision would have to be complied with versus the routine "16 court day" notice required for pleading motions.
The special Motion to Strike, known as the Anti-SLAPP (Strategic Lawsuit Against Public Participation), however, is (or should be) supported by evidence, such as written discovery, declarations, depositions, etc. It is an evidentiary motion.
And filing one has some chilling effects for unprepared plaintiffs. Filing the motion freeezes all discovery in the case. The plaintiff cannot rely on the allegations in the Complaint, even if well pled. The plaintiff has to produce admissible evidence that creates triable issues of fact if the Complaint "arises from" free speech or other protected activity, such the right to petition, i.e., court litigation, administrative litigation involving public agencies, etc.
Courts thus analyze two "prongs" of the Anti-SLAPP Act. First, does the cause of action "arise from" speech or protected activity? That might seem easy, but if a cause of action is "mixed," then the court has determined if the "gist" (translation: 50%+) of the cause of action arises from protected activity, the first prong is met. If the gist of the cause of action does not arise from protected speech or protected activity, the motion is denied.
Two things are worth noting here. One, if the entire cause of action cannot be disposed of under either the first or second prong, the motion must be denied, unlike the ordinary Motion to Strike, which can attack part of a cause of action. And, only if the Anti-SLAPP Motion to Strike is objectively frivolous, the prevailing plaintiff cannot recover fees; whereas a prevailing defendant must be awarded reasonable fees related to the Anti-SLAPP Motion.
Assuming the first prong is met, the second question the court must answer is whether plaintiff has presented sufficient admissible evidence to create triable issues of fact on the SLAPP claim. If the moving defendant offers no evidence, plaintiff's burden is much easier. If there is evidence, however, or if there are applicable statutory privileges, such as California's Litigation Privilege found in Civil Code, § 47(b), the plaintiff is in trouble. Absent admissible evidence sufficient to create triable issues of fact or absent law showing an exception to a statutory bar like the Litigation Privilege, the defendant wins the Anti-SLAPP Motion, and is awarded fees for defending against the SLAPP claim. Sometimes, a well-drafted Anti-SLAPP Motion to Strike will knock out an entire Complaint, resulting in an immediate judgment.
For the defense bar, the Anti-SLAPP Motion to Strike is something to think about when responding to an initial Complaint. If not brought within the first 60 days of being served, courts have discretion to deny the motion, since these motions are supposed to be brought early. There is an immediate right to recover fees, and the motion for fees may (and usually should) follow the Anti-SLAPP Motion to Strike itself as after the motion is granted, it is easier to know the amount of fees and costs incurred in bringing the motion. Also, the California Supreme Court has interpreted the Anti-SLAPP Act as authorizing the recovery of fees for having to bring a subsequent motion to recover attorney's fees, that is, "fees on fees" are permissible under the Act.
The above is an overview of an ever-changing and evolving area of law in California, though several other states have enacted Anti-SLAPP laws similar to California. It is generally a good idea to get counsel who are experienced in this area of law, given the potential risks of fee exposure, etc.