Boyd's student from 1924 to 1927). Like the British practitioners, Spolin felt that playing games was a useful means of training actors and helped to improve an actor's performance. With improvisation, she argued, people may find expressive freedom, since they do not know how an improvised situation will turn out. Improvisation demands an open mind in order to maintain spontaneity, rather than pre-planning a response. A character is created by the actor, often without reference to a dramatic text, and a drama is developed out of the spontaneous interactions with other actors. This approach to creating new drama has been developed most substantially by the British filmmaker Mike Leigh, in films such as Secrets & Lies (1996), Vera Drake (2004), Another Year (2010), and Mr. Turner (2014). Improvisation is also used to cover up if an actor or actress makes a mistake. Physiological effects Acting in front of an audience many times can cause "stage fright", a form of stress in which someone becomes anxious in front of an audience. This is common among actors, especially new actors, and can cause symptoms such as increased heart rate, increased blood pressure, and sweating. In a 2017 study on American university students, actors of various experience levels all showed similarly elevated heart rates throughout their performances; this agrees with previous studies on professional and amateur actors' heart rates. While all actors experienced stress, causing elevated heart rate, the more experienced actors displayed less heart rate variability than the less experienced actors in the same play. The more experienced actors experienced less stress while performing, and therefore had a smaller degree of variability than the less experienced, more stressed actors. The more experienced an actor is, the more stable their heart rate will be while performing, but will still experience elevated heart rates. Semiotics The semiotics of acting involves a study of the ways in which aspects of a performance come to operate for its audience as signs. This process largely involves the production of meaning, whereby elements of an actor's performance acquire significance, both within the broader context of the dramatic action and in the relations each establishes with the real world. Following the ideas proposed by the Surrealist theorist Antonin Artaud, however, it may also be possible to understand communication with an audience that occurs 'beneath' significance and meaning (which the semiotician Félix Guattari described as a process involving the transmission